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New tools are revealing the crowd behavior of electrons at close to absolute zero, where freezing and melting are governed by quantum mechanics
If you cool a piece of ice to the very lowest temperatures possible, about the last thing you would expect it to do is melt. But physicists are learning that some exotic crystals made of electrical charges or electron spins do indeed engage in something like that odd behavior: They "melt" at absolute zero, changing from one phase to another. These phase transitions are strictly in the weird realm of quantum mechanics, a world dominated by large fluctuations in energy and momentum even at the lowest temperatures, where classical physics would insist that the opportunity for change is frozen.
Theorists have been exploring these transitions since the 1950s, but now experimenters are actually seeing these strange metamorphoses in the laboratory. With sophisticated tools for building semiconductors and improvements in low-temperature analysis, physicists have been able to watch the melting of an electron crystal and the unusual flipflops of two-dimensional "gases" of electrons. This new kind of crowd behavior is fascinating in its own right, they say. "These are things that just can't be discussed in terms of single particles at all," says Subir Sachdev, a Yale University physicist. But it may also have some practical implications: By searching for hints of quantum phase transitions in hightemperature superconductors, researchers are hoping to spring the lock on the stubborn mystery of how these materials work.
Classical phase transitions, like the melting of an ice cube, are driven by thermal energy. Heating the ice above the freezing point causes molecules in the solid water to vibrate. Eventually, the jiggling becomes so wild that the molecules are no longer content with their orderly seating arrangement in the ice crystal, and they break loose to become liquid water. Heat them even more, and they don't even want to slosh around in a liquid-they vaporize into steam. Reverse the process, the jiggling slows down, and the molecules fall back into place. The variable that controls all of these transformations is temperature.
Quantum phase transitions are a completely different beast. What opens the way to a quantum phase transition is a change not in temperature, but in some...